1. Field of the Invention
Example embodiments of the present patent application relate to a document feeding device, an image forming apparatus including the document feeding device, and a control method for the document feeding device, and more particularly, to a document feeding device that conveys multiple documents by feeding one by one to a document reading device, an image forming apparatus including the document feeding device, and a control method of the document feeding device for controlling sheet feeding of the documents.
2. Discussion of the Related Art
In so-called optical character recognition (OCR) technology, in which a scanner reads an image of a sheet-like document, for example, to convert the image into electronic data to be used as text or image data, and in a so-called copying technology, which makes a copy of a sheet-like document, for example, a document feeding device including a separation mechanism is often used. The document feeding device feeds and conveys a plurality of documents by automatically and sequentially extracting and separating the documents one by one from a document bundle.
As an example of such a document feeding device, there is a background document feeding device that draws documents from a document bundle, separates only the topmost document to be read from the other documents, detects whether or not there is a next document, and repeats the operation of drawing and separating documents until there is no next document.
In recent years, a demand for high-speed processing capability of this type of document feeding device has become particularly prominent due to an increasing need for OCR technology and a demand for increased productivity, for example. To satisfy such demand for a high-speed processing capability, techniques for increasing the document reading speed and reducing the intervals between documents read in the reading operation have been developed. Accordingly, there is also an increasing need for high-speed operation, in which documents to be read are reliably separated and fed one by one from a document bundle.
In the document separating operation, however, only the first document to be fed is conveyed, and the second and subsequent documents are prevented from being fed or in some cases are conveyed in the reverse direction. Therefore, mutually conflicting operations are required for the first document (hereinafter occasionally referred to as the previous document, for convenience sake) and the second or subsequent document (hereinafter occasionally referred to as the next document, for convenience sake). As a result, the positioning of the next document varies and becomes unpredictable after the completion of the feeding operation of the previous document.
To increase the speed of the separating operation, it is desirable to feed the next document as soon as possible after the trailing end of the previous document passes a predetermined sensor position. However, due to the unpredictable separating operation, i.e., due to the varied and unpredictable position of the next document after the conveyance of the previous document, it is difficult to efficiently control the feeding timing by predicting the position of the next document, i.e., the second or subsequent document. This difficulty presents a major obstacle to improving the high-speed separating operation.
To facilitate an understanding of the matter, the above-described phenomenon will now be described in greater detail with reference to FIGS. 1A to 2D. FIGS. 1A to 1F are enlarged schematic cross-sectional views illustrating an example of a separation mechanism of the background document feeding device. FIGS. 2A to 2D are schematic cross-sectional views illustrating a variety of leading end positions of the next document after the feeding of the previous document in the separation mechanism of the background document feeding device. The drawings illustrate a document bundle 1, a pickup roller 7, a document set sensor SN5, a feeding belt 9, a separation roller 10, a document detection sensor SN2, a document contact amount detection sensor 11, and a pair of pullout rollers 12.
FIG. 1A is a schematic cross-sectional view illustrating an enlarged view of the separation mechanism of the document feeding device, wherein the set document bundle 1 is going to be fed. In this example, the document set sensor SN5 first detects that the document bundle 1 has been placed on a document table serving as a carrying unit. In accordance with the detection of the document bundle 1, the pickup roller 7 descends to come into contact with the topmost surface of the document bundle 1, and rotates in the clockwise direction in the drawing to feed a document between the feeding belt 9 and the separation roller 10, and area that is hereinafter referred to as a document separation nip portion.
In this conveying operation, the feeding belt 9 stretched over belt pulleys with a predetermined tension, for example, is also rotated in the clockwise direction in synchronization with the rotation of the pickup roller 7 as the belt pulleys rotate. Due to the rotating operation of the feeding belt 9, the document (hereinafter referred to as the document P) is conveyed to the document separation nip portion as illustrated in FIG. 1B. In this process, the feeding belt 9 is pressed against the separation roller 10, which is provided to face the feeding belt 9 at the document separation nip portion as described above, at a predetermined pressure. Meanwhile, the separation roller 10 is frictionally driven via a torque limiter, not illustrated, having a predetermined amount of set torque. When the separation roller 10 is in direct engagement with the feeding belt 9 or in engagement with the feeding belt 9 with the document P interposed therebetween, the separation roller 10 is rotated in the counterclockwise direction in the drawing in accordance with the rotation of the feeding belt 9. However, the force for rotating the separation roller 10 in accordance with the rotation of the feeding belt 9 is set to be smaller than the torque of the torque limiter when two or more documents enter the document separation nip portion. Therefore, the separation roller 10 is configured to rotate in the clockwise direction, which is the original driving direction, to push back the extra document, i.e., the next document, to prevent a plurality of documents from being conveyed at the same time.
As the single document, i.e., the previous document P separated from the other documents in FIG. 1B is further conveyed, the leading end of the previous document P reaches the position of the document detection sensor SN2 as illustrated in FIG. 10. Triggered by the leading end of the previous document P reaching the document detection sensor SN2, the pickup roller 7 ascends to complete the separating operation. It is not necessary that the ascent of the pickup roller 7 coincide with the arrival of the previous document P at the document detection sensor SN2. Thus, a slight time lag may be set.
Even after the pickup roller 7 is lifted out of the way in the above-described sequence of operations, the feeding belt 9 still continues to rotate. Therefore, the previous document P separated from the document bundle 1 is further conveyed by the feeding belt 9. Then, as illustrated in FIG. 1D, the leading end of the previous document P enters between the pair of opposed pullout rollers 12 (an area that is hereinafter referred to as a pullout nip portion), which are in contact with each other but have not yet started rotating. In this process, after the leading end of the previous document P passes the document contact amount detection sensor 11, the feeding belt 9 is further driven for a time period determined by the contact amount. As a result, the previous document P is rolled up against the pullout rollers 12 with a predetermined force. Due to the action of the previous document P attempting to return to its original shape, which is caused by this bending of the previous document P, the leading end of the previous document P is ultimately positioned in front of the pullout nip portion formed by the pair of pullout rollers 12.
Thereafter, the pair of pullout rollers 12 start to be driven, as illustrated in FIG. 1E, thereby further conveying the previous document P, as illustrated in FIG. 1F, to a document reading device that is not illustrated in FIGS. 1A to 1F.
According to the above-described configuration of the related-art document feeding device, even if the trailing end of the previous document P is normally extracted from the document bundle 1, the passage of the trailing end of the previous document P is detected, at the earliest, after the trailing end of the previous document P passes the document detection sensor SN2. Accordingly, the feeding of the next document starts only after the trailing end of the previous document P passes the document detection sensor SN2. In a state in which the pair of pullout rollers 12 is moving, as illustrated in FIGS. 1E and 1F, the feeding belt 9 is not driven. However, with the use of a one-way clutch and so forth, for example, the feeding belt 9 is configured to be rotated in accordance with the conveying operation of the previous document P.
As described above, even if the trailing end of the previous document P is normally extracted from the document bundle 1 set on the document table or passes the document separation nip portion, it is difficult for the above-described configuration of the background document feeding device to promptly detect the passage of the trailing end of the previous document P through the document bundle 1 or the document separation nip portion. Therefore, a signal necessary for starting the feeding of the next document (hereinafter referred to as the next document P′) is not obtained. As a result, the feeding of the next document P′ is not started. The feeding of the next document P′ starts only after the trailing end of the previous document P passes the document detection sensor SN2. This state of affairs is not conducive to improving high-speed operation.
In addition, when the trailing end of the previous document P passes the document detection sensor SN2 and the conveying operation of the next document P′ is about to start, the position of the leading end of the next document P′ is moved forward somewhat in some cases during the conveyance of the previous document P for reasons that are described below. Consequently, the position of the leading end of the next document P′ is inconstant and unpredictable. For example, the leading end of the next document P′ may stay substantially at the initial document set position, as illustrated in FIG. 2A, or may be conveyed to a position in front of the feeding belt 9, as illustrated in FIG. 2B. Further, the leading end of the next document P′ may reach the document separation nip portion formed by the feeding belt 9 and the separation roller 10, as illustrated in FIG. 2C, or may be located inside or project from behind the document separation nip portion, as illustrated in FIG. 2D.
The above phenomenon is attributable to frictional force interfering with movement in opposite directions, which acts between the previous document P and the next document P′, i.e., frictional force generated in the feeding of the previous document P and resultantly affecting the next document P′. Further, if the next document P′ has reached the document separation nip portion during the operation of separating the previous document P from the document bundle 1, the next document P′ may also be subjected to the action of the separation roller 10 to push the extra document backward. Therefore, it is very difficult to predict and control the point at which the movement of the next document P′ stops, i.e., the behavior of the leading end of the next document P′, from the conveying operation of the previous document P.
In addition, users use a variety of different types of document sheets. Therefore, the coefficient of friction of the documents can be expected to vary depending on the type of documents used. Further, the coefficient of friction of the separation roller 10 also varies due to retention of paper particles and so forth from the documents. Due to the variation in the coefficient of friction caused by the difference in sheet type and/or the paper particles and so forth of the documents, therefore, the frictional force and the reverse conveying force applied to the next document P′ can also be expected to vary. From this perspective also, then, it is understood that the prediction and control of the position of the next document P′ is substantially difficult.
It is conceivable that the position of the document detection sensor SN2 may simply be shifted to the upstream side in the document conveying direction to detect the passage of the trailing end of the previous document P as soon as possible. However, the position of the leading end of the next document P′ varies, as described above. Accordingly, if the next document P′ is conveyed partially overlapping the previous document P, and if the leading end of the next document P′ reaches the document detection sensor SN2 shifted to the upstream position, the document detection sensor SN2 might fail to detect the boundary between the previous document P and the next document P′. As a result, a so-called conveyance jam is caused by abnormal retention of the documents.
In terms of controlling an image forming apparatus, in an attempt to increase productivity by optimizing the start timing of the feeding of the next document P′, the above-described failure to reliably detect the variable position of the leading end of the next document P′ and precisely separate the next document P′ in the next separating operation is viewed as a so-called conveyance jam occurring. Therefore, in the configuring of the related-art document feeding device, for reasons of safety it is necessary to implement such control assuming the shortest document interval between the previous document P and the next document P′, as illustrated in FIG. 2D. That is, if it is not certain that the trailing end of the previous document P has passed the document detection sensor SN2, it is not allowed to perform the control of starting the feeding of the next document P′, the leading end of which might have reached the document separation nip portion. In fact, however, an unnecessarily long document interval is set between the previous document P and the next document P′ in the above-described control of the background document feeding device in consideration of safety, because in practice there are cases like that illustrated in FIG. 2A, in which the control of starting the feeding of the next document P′ can be performed before the document detection sensor SN2. Therefore, such overly conservative control presents an obstacle to increased productivity.